Microsoft suffered a stunning defeat on Monday when a European Union court backed a European Commission ruling that the US software giant illegally abused its market power to crush competitors. The European Union's second-highest court dismissed the company's appeal on all substantive points of the 2004 antitrustruling. The court said Microsoft, the world's largest software maker, was unjustified in tying new applications to its Windows operating system in a way that harmed consumer choice. The verdict, which may be appealed only on points of law and not of fact, could force Microsoft to change its business practices.